Period Calculator
Predict your next three periods and estimated ovulation day from your last period and average cycle length.
Reviewed October 7, 2026 Runs in your browser — inputs are never stored
This tool provides general educational estimates only and is not medical advice. It does not diagnose, treat, or replace guidance from a qualified healthcare provider. Consult a doctor for decisions about your health.
About the Period Calculator
The period calculator predicts the start and end dates of your next three periods and your estimated ovulation date. It adds your average cycle length to the first day of your last period for each upcoming cycle and uses your typical period length to estimate the end date. Ovulation is estimated 14 days before the next period. Predictions are most reliable when your cycles are regular; stress, illness, travel and hormonal changes can shift them. It is not a method of contraception.
How to use it
- 1Select the first day of your last period.
- 2Enter your average cycle length in days.
- 3Enter how many days your period usually lasts.
- 4Press Calculate to see your next three periods and estimated ovulation date.
Worked example
with a last period starting on 1 March, a 28-day cycle and a 5-day period, the next periods are expected on 29 March–2 April, 26–30 April and 24–28 May. Ovulation is estimated for 15 March.
How it works
How it works
Each period start is your last start plus multiples of your cycle length. Ovulation is estimated 14 days before the next period.
Who it is for
Anyone who wants to plan around their menstrual cycle.
Why it helps
Knowing when your next periods are due makes it easier to plan travel, events and supplies.
Common use cases
- Planning holidays and events around your cycle
- Noticing when a period is late
- Tracking cycle regularity over several months
Frequently asked questions
A:Predictions assume a regular cycle; irregular cycles are harder to forecast.
A:No. Cycle tracking is not a reliable form of birth control.
Reviewed for accuracy · Last updated October 7, 2026
Formulas follow recognised clinical references — including WHO BMI classifications, the Mifflin-St Jeor equation, and Naegele’s rule — and are periodically checked against published reference values.
CalcWisee Editorial Team
Calculator development and formula review
CalcWisee is an independent site founded by Ayman. Every calculator is built from a published formula or official definition, tested against hand-worked examples before it goes live, and corrected promptly when a reader reports an issue to ayman@calcwisee.com.
Expertise: Financial mathematics, Health and fitness formulas, Applied mathematics, Unit conversion standards
References & sources
Related guides
Understanding BMI and a Healthy Weight Range
Learn how BMI is calculated, what each category means, a worked example, and the real limits of BMI as a health measure.
BMI Calculator: Check If Your Weight Is Healthy
Your weight alone does not tell the whole story. Use the CalcWisee BMI Calculator to calculate your Body Mass Index, find your weight category, and understand what it means for your health.
BMR Calculator: Calculate Your Daily Calorie Needs
Understand how many calories your body burns at rest and estimate your daily calorie needs with the free CalcWisee BMR Calculator — using the Mifflin–St Jeor Equation.